An extruder head for extrusion of a thermoplastic product made from a base plastic material and at least one marbling additive material is connectable to a plastic extrusion screw press having a cylindrical housing and a screw shaft.
This extruder has a mixer connected to the screw shaft and a mixer housing connected to the cylindrical housing. The mixer housing has a device for feeding the additive material and the mixer has a connector piece whose diameter corresponds to that of a core of the screw shaft associated therewith.
By "marbling" or "veining" we mean the formation of an intermixture of two or more thermoplastic materials such that the extruded product has a marbled structure, i.e. visible veins of the additive within the base material and hence an appearance that is inhomogeneous to the eye in cross section and along the periphery--usually through and through. Such extruded products are used for a variety of purposes, for example as bars, legs, arms, sections or strips in the furniture industry in situations where the section or strip should have a woodlike grain when originally visible or after shaping or working in connection with an article of furniture.
An extruder for making a thermoplastic product from a base plastic material and at least one additive marbling material is described in German Utility Model DE-GM 85 30 358. In the known process however the additive material is brought into an extruded stream made of the base plastic material from the outside of the extruded stream in the extruder.
This leads to laminations which depart seriously from the marbling. To ensure a marbling effect an additional agitation or turbulence must be introduced; this frequently is unsatisfactory. In the known extruder the mixer comprises a plate which is inserted diametrically and axially parallel in a connector piece for the mixer.